Anti-ballistic missile guidance systems require extremely accurate warhead fuzing. A radar fuze system on a missile uses very short pulse width and extremely fast rise/fall time RF pulses to determine the range and range-rate of the target. A receive/transmit (RX/TX) system is employed to process the RF radar pulse and its target return. In a tactical mode, radar pulses are amplified by a high-power transmitter, then broadcast at the target. The target returns the pulse, which enters the transmitter/receiver through the antenna, and is amplified and down-converted by the receiver.
For range calibration, the system should provide an alternate signal path from the transmitter to the receiver, which bypasses the antenna, to enable the system to measure its own time delay. In a calibration mode, the alternate signal path should maintain three characteristics. First, it should attenuate the radar pulse signal power dramatically to within a small amplitude window to simulate the small radar return of a target with an acceptable IF output power, while keeping the receiver well below its saturation level but above the system noise floor. Second, it should preserve the integrity of important radar pulse characteristics, including rise/fall times and pulse width. Third, it should provide a group delay nearly identical to the group delay of a transmitted/received pulse reflected from a theoretical target at a fixed close range. The tolerance versus operating frequency, temperature, exciter power, and from unit-to-unit on the group delay difference between the calibration mode signal path and the tactical mode signal path sets the accuracy to which a target""s range can be determined.
A switchable two-junction circulator has been used to establish a calibration mode path. The first junction was used to attenuate the transmitter output power during the calibration mode, while the second junction bypassed the antenna during calibration and routed the radar pulse directly to the receiver. This approach allowed the transmit amplifier chain to operate at full output power during calibration, thereby preserving the important pulse characteristic of rise/fall time and pulse width. However, the poor frequency response of the first circulator junction in the calibration mode had an undesirable effect on pulse rise/fall time, since the dominant signal path through this junction in the calibration mode is in the reverse direction of normal signal flow. This reverse-direction signal path has a frequency response similar to that of a notch filter, containing large variations in signal amplitude and group delay over the range of frequencies represented by the pulsed RF signal. This lack of amplitude and delay flatness in the frequency domain translates to poor signal fidelity (e.g. changes in pulse rise/fall time, overshoot, and ringing) in the time domain. A pair of SPST switches within the receiver chain further reduced the calibration mode signal amplitude within the receiver, but had an unpredictable group delay variation due to leakage path effects, resulting from the dominant signal path in the receiver in the calibration mode including the leakage through these SPST switches in their OFF positions. These switches have no requirements for amplitude or delay flatness with respect to frequency in their OFF positions. Typically, in the OFF position, these switches exhibit large variations in amplitude and group delay with respect to frequency. Similar to the reversed circulator junctions, the amplitude and delay variations that these switches exhibit in the frequency domain translate to poor signal fidelity in the time domain.
A prior approach by applicants incorporated a bypass path on the transmitter, instead of switching the first junction of the two-junction circulator to attenuate the transmit pulse amplitude. This scheme provides improved group delay accuracy, but does not accurately preserve pulse rise/fall times and pulse width because the transmit chain""s non-linear elements are operating at two radically different conditions in the tactical and calibration modes. The accuracy of this approach was found to be highly dependent on temperature and input power. The addition of a controlled attenuation bypass path in the receiver improved the group delay and frequency response characteristics of the RX/TX.
An exemplary embodiment of a system in accordance with the invention is a radar system having a tactical mode and a calibration mode. A transmitter section provides high-power amplification of an RF pulsed waveform from an exciter during the tactical mode and the calibration mode. A three-junction circulator system has an input port connected to an output of the transmitter section, and includes a high-power attenuator. The circulator system provides a transmit tactical mode signal path and a transmit calibration mode signal path of virtually identical electrical path lengths for a transmitter output signal. The tactical path passes through the first, second, and third junctions in a first direction to an antenna I/O port. The calibration path passes through the first, second, and third junctions in a second direction and through the high-power attenuator to an output port. A receiver section is coupled to the output port, and includes a receive tactical mode signal path through a low noise amplifier (LNA), mixer, and intermediate frequency (IF) amplifier to an IF output port for down-converting a radar signal. The receiver section further includes a receive calibration signal path through an attenuator which bypasses the LNA and passes through the mixer and IF amplifier to the IF output port.